Turbine Oil Analysis

A properly maintained lubrication system is key for effective performance and reliability of a turbine power system.

Turbine oil is required to support journal and thrust bearings under moderate and heavy loads and elevated temperatures with an oil film that is thinner than a human hair. The oil in these systems is expected to provide trouble free service life for 5 to 15 years, depending on the system. Cleanliness and good oxidation resistance is critical to the performance of these lubricants

The three types of industrial application turbine power systems (non-aviation) are gas, steam, and hydroelectric.  Systems can vary in size and design, reservoir configuration, and rotational speeds that affect the life and performance requirements of the lubricants in service. Turbine oils are expected to lubricate and protect components such as journal and thrust bearings, gears and gear drives. They are also required to provide power in directional and hydraulic controls components, such as governors, control valves, and safety devices.  Cooling of lubricated parts is also an essential function of in-service turbine oils.

Type	Shaft Speed, rpm	Oil Temperature, C (F)	Oil Service Life, Years
Gas	3,000-9,000	55-95 (130-200)	3-5
Steam	1,000-20,000	45-70 (110-160)	10+
Hydroelectric	75-900	45-70 (110-160)	10+

 

Turbine lubrication systems should be inspected on a regular basis. Testing should be performed according to a routine or monthly schedule, quarterly schedule, and annual schedule. Routine testing can inspect for lubricant cleanliness, contaminants, and general product integrity, as well as component wear. Quarterly and annual testing will look at oxidative and service life performance, varnish potential in the system, as well as routine tests for cleanliness, contaminants, general product integrity, and component wear. 

Routine monitoring should also include a check list of visual inspections that are to be performed. This can include the reservoir, oil purification components (filters, centrifuge), seals, leaking fittings, water drains, and sight glass. Noting the smell of turbine oil can also detect possible contamination or degradation of the oil. If the oil has a distinct sulfur (rotten egg) smell, it can be an indication of microbial contamination in the system. If the odor becomes pungent or sour smelling, it could be an indication of overheating from the oil cracking due to excessively high operating temperatures. Some testing specific to turbine application and design can be performed as needed or included in an inspection schedule. Water separability and rust inhibitor performance in steam and hydroelectric systems should be periodically measured. Because of the nature of the severe service environment in gas turbine systems due to high temperatures and mixture with exhaust gases, monitoring oxidation and varnish potential is critical. Testing for foaming tendency may be required if excessive foaming occurs due to system design or contaminants in the fluid.

As mentioned, cleanliness is important to turbine oil circulating systems. Monitoring for water contamination is important. Generally water should be kept at less than 200 ppm. Above this level bearing wear greatly increases. High pressure and low clearance components in a circulating system are susceptible to increased wear and blockage due to particulate contamination. Contaminants can enter the system due to air leaks, poorly maintained breathers and reservoirs, external environmental conditions, topping off with added oil, and internally generated from wear and oil degradation.

Compatibility testing of lubricants is recommended when changing from one product formulation to another, especially when changing from Type I based stock to Type II. Mixing incompatible oil formulations can dilute or make additive chemistry ineffective, as well as create insoluble soaps that plug passageways and facilitate additive dropout.

ALS can work with your maintenance team and lubricant supplier to ensure the right testing is preformed to maximize your equipment life and reliability during years of service. For further information and assistance with establishing reliability goals contact one of our certified client support staff. 

Written By:

David Doyle, CLS, OMA I, OMA II
General Manager, ALS Tribology